update sample to work with firefox (no auto instanciated id)

Despite the "spec", firefox cannot handle the id-to-global-variable in the demo.html.

I found some resources speaking about this spec I did not know:

- https://stackoverflow.com/questions/6381425/is-there-a-spec-that-the-id-of-elements-should-be-made-global-variable
- http://www.2ality.com/2012/08/ids-are-global.html

What do you think?